Transcript Rarity and Extinction

Rarity and Extinction
Biology/Env S 204
Spring 2009
Rarity
• What makes something rare? Why should
we care?
• Rarity (presence in small numbers) is
common—the vast majority of species are
present as relatively few individuals
• But species can be rare in different ways
• These differences have implications for
vulnerability to extinction and for
protection
Rarity
It is common
to be rare!
Rarity
• There are at least 7 ways for a
species to be rare
• E.g., due to habitat specificity, very
restricted geographic range, small
population sizes, or some combination
• Very localized species are vulnerable
to habitat destruction or chance
events
Extinction
• Ca. 300 years ago (late 1600’s) there
were an estimated 3-5 billion
individuals of the passenger pigeon
• Native to eastern North America
• Accounted for ca. 25% of all North
American land birds (total number of
individuals)
• Was one of the most abundant
globally
Extinction
• The passenger pigeon was relatively
abundant until 1871, then it declined
rapidly until 1880 and became scarce
• The last known attempt at colonial nesting
was in 1887 and by the 1890’s the bird
was very rare
• The last wild birds were taken in 1899,
1900
• Last individual died in 1914 at the
Cincinnati Zoo (a female named Martha)
Extinction
• What happened?
• This species was nomadic with colonial
nesting, grouped into few, very large
traveling groups which then subdivided into
smaller colonies for nesting
• But even “smaller” colonies contained
hundreds of millions of individuals
• Depended on mast fruiting of oaks, beeches
and chestnuts; pigeons were part of the
natural disturbance regime
Extinction
• The birds were regarded as a pest
but they were also hunted for food
• Harvesting for local use did not
appear to have much impact though
• Traditional harvesting involved the use
of nets (up to 100 birds at a time),
which did not disturb nesting
• Use of guns led to nest abandonment
Extinction
• Deforestation in the East began to affect
those pigeon populations by the mid-1800’s
but still major forest areas were left in
Michigan, Ohio, Wisconsin, etc.
• In 1830, there were only 23 miles of train
tracks in the United States
• By the Civil War, there were 30,000 miles
of tracks and telegraph technology
• Market harvesting was now feasible
Extinction
• Largest documented harvest was in
Michigan in 1878: the nesting colony
covered 250 square miles and in 3
months, at least 1.5 million dead
birds and over 80,000 live birds were
shipped by rail and by water to large
cities for food
• Direct harvest of nestlings and nest
abandonment led to steep decline
Extinction
• Passenger pigeons could breed in captivity
and some nested solitarily in the wild
• But the protection and genetic diversity of
large populations was lost
• Habit of nesting in trees with unconcealed
nests and only one egg per season was
insufficient in a species whose reproductive
strategy depended on mass nesting
Extinction
“The loss of the passenger pigeon
vividly demonstrates that there is no
species whose numbers are so great
as to be immune to extinction.”
E. O. Wilson &
D. Perlman, 2000
Extinction—General
• Extinction is a normal process
• Diversity at local to global scales is
determined by rates of speciation and
extinction
• Data on past extinctions (and rates)
are from fossils based on assumptions
that may not be accurate
Extinction—General
• For the current rate of extinction, we
don’t how much diversity there is for
many groups, so we can’t be sure of
what we are losing
• Most measured at the species level;
for snails, birds, mammals the data
are pretty good
• Need to have some kind of objective
measure
Extinction—
How does it happen?
1) Direct elimination (usually on a short
time scale) by outright destruction
or catastrophic events
2) Erosion of genetic variability in
various ways that ultimately result in
the inability to produce progeny and
the death of individuals
Extinction—
How does it happen?
3) Pre-human appearance, natural
disturbances/catastrophes/processes
were responsible
4) Presence of humans adds another
factor; human activities and the
consequences are responsible for the
vast majority of modern extinctions
Extinction—
How does it happen?
• To some extent can predict which
species will be most vulnerable:
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Rarity
Dispersal ability
Specialization
Population (genetic) variability
Trophic level
Adult survival rate
Extinction Rates
• About 60% of extinctions occurred in
a number of relatively short episodes
= mass extinctions
• Therefore, extinctions have been
relatively low for the remainder
• Background rate is difficult to
calculate, but the average lifespan of
a species in the fossil record is ca. 4
million years
Extinction Rates
• If we assume total species diversity at 10
million, this suggests a background rate of
2.5 species per year
• But the fossil record may be biased toward
successful, often geographically wideranging species, which probably survive for
longer than 4 million years
• Therefore a lot of species will survive for
shorter periods, so background rate is
probably higher
Extinction Rates
• Even if we assume a rate 10 times higher
and apply it to modern taxa, we would
expect 1 of the 4,000+ mammal species to
go extinct every 400 years and 1 of the
8,000+ bird species to go extinct every
200 years
• Extinction rates in recent times have been
far higher than this for these groups; basis
of the argument that currently humans are
causing a mass extinction event
Mass Extinctions
• Within the last 600 million years,
there have been 5 mass extinction
events
• Most severe was the Permian
extinction about 245 million years
ago; estimated that 96% of species
alive then disappeared
Mass Extinctions
• The Permian extinction took place over 5-8
million years, associated with continental
movements (formation of Pangaea), climate
change, increased volcanic activity
• In contrast, the Cretaceous extinction 65
mya was of shorter duration, highly likely
that the cause was a meteor impact
• Less significant for marine life, much
greater impact on terrestrial organisms
• Not clear if elevated rates of speciation
follow mass extinctions or if survival rate
of fledgling species increases
Current Extinction
• First significant impact of humans on
the world’s biota happened within the
last million years
• Humans arrived in Australia about
50,000 years ago; lost nearly all of
its species of very large mammals,
giant snakes and reptiles and nearly ½
of its flightless birds around this time
Current Extinction
• Humans arrived in North and South
America at least 11,000 years ago;
lost about ¾ of the genera of large
mammals at about this time
• But, some argue that climatic factors
were responsible
• No question that the European Age of
Expansion in the 15th & 16th centuries
initiated another wave of extinctions
Current Extinction
• But extinctions definitely occurred before
this time: Polynesians were apparently
responsible for exterminating 50 of about
100 endemic bird species in the Hawaiian
Islands before Europeans arrived in the
18th century
• Similar impact in New Zealand, colonized
500 years later than Hawaii; moas went
extinct before Europeans arrived
Current Extinction
• Two major trends within the last 400
years:
– Documented island extinctions began
almost two centuries earlier than
continental extinctions and
– Both island and continental extinctions
have increased rapidly from the early or
mid-19th to the mid-20th century (see
Brown-eyed and Milk-giving in text)
Commitment to Extinction
• Extinction is usually attributed when a
species has not been recorded (sighted or
collected) over a significant time span
• Convention on Trade in Endangered Species
(CITES) has set an arbitrary time of 50
years
• The longer a species has not been
recorded, the more likely it is to be
extinct
Commitment to Extinction
• Habitat destruction/modification
(leading to fragmentation) and invasive
species are the most serious current
threats to biodiversity; they are the
primary causes of current extinctions
• Many species still present are
probably so heavily impacted by these
processes that they are “committed
to extinction”
Commitment to Extinction
• Remember the estimated background
rate of 2.5 species/year?
• Current estimates predict that we
could lose up to 50,000 species/year,
20,000 times the background rate
• By the year 2100, as much as twothirds of Earth’s biota will have
disappeared or be committed to
extinction
Iowa Pleistocene Snail
Originally known only from the fossil record;
thought to have been extinct for 10,000 years;
rediscovered in NE Iowa in 1980.
Endangered Species Act
• ESA signed into law by Richard M. Nixon in
1973
• “…to provide a means whereby the
ecosystems upon which endangered species
and threatened species depend may be
conserved, to provide a program for the
conservation of such…species and to take
steps…to achieve the purposes of the
treaties and conventions set forth.”
Endangered Species Act
The snail darter is probably the species most closely
associated with the ESA. It stopped the Tellico
Dam but later additional populations were found
and it was downgraded to threatened status.
Endangered Species Act
• Deals mostly with species and species
habitat, does not protect ecosystems
supporting multiple habitats and species
• Provides ability to slow or even stop
development
• Vertebrates receive higher levels of
protection than invertebrates and plants
• But also has generated controversy,
especially with regard to private property
rights
Endangered Species Act
• Re-authorized in 1978, 1982, 1988,
and 1992 but not since
• Does not adequately address habitat
fragmentation
• Amendments/bills to provide a selffunding mechanism have been
unsuccessful so far